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Status Reproduced on a release build of main 5c6d4e2 with a local mock S3 ( const writer = s3.file("obj.bin").writer({ partSize: 5 * 1024 * 1024 });
writer.write(new Uint8Array(5 * 1024 * 1024));
writer.write(new Uint8Array(100));
await writer.end(new Error("source failed mid-stream"));
The tests are the block One open question for a maintainer is at the top of the Notes in the description: whether |
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Updated 4:43 AM PT - Oct 23rd, 2026
✅ @robobun, your commit 293e060c9a92a2e45cb79dd4b4f1926c64bade39 passed in 🧪 To try this PR locally: bunx bun-pr 43134That installs a local version of the PR into your bun-43134 --bun |
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Walkthrough
ChangesS3 error termination
Priority: ➖ Normal Merge Risk: ⚪ Minimal · up to
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Check skipped because no linked issues were found for this pull request. |
| Out of Scope Changes check | |
Check skipped because no linked issues were found for this pull request. |
| Title check | |
The title clearly states the primary change: S3 writer uploads no longer complete when end() receives an error. |
| Description check | |
The description follows the required template with both sections present: "What does this PR do?" (titled "Problem", "Fix", "Background", "Downsides") and "How did you verify your code works?" (covere… |
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Explanation
The description follows the required template with both sections present: "What does this PR do?" (titled "Problem", "Fix", "Background", "Downsides") and "How did you verify your code works?" (covered under "Verified" and the Notes "Tests" section, which lists the suites run, the mutation check, the failing-on-main counts, and the clippy/check results). The problem, fix, behavior changes, and verification are specific and tied to concrete evidence. Minor gaps: the template's section headings are not used verbatim, and the "What does this PR do?" content is spread across several headings rather than one summary. These are non-critical, so the description is mostly complete.
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Actionable comments posted: 2
- 🪄 Fix CodeRabbit comments on this PR
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Inline comments:
In `@packages/bun-types/s3.d.ts`:
- Around line 92-93: Update the JSSink::js_end documentation in the S3 type
declaration to clarify that only Error instances use the error path; any other
value performs normal completion through NetworkSink::end_from_js, completing
the upload and storing its data like end() without an argument.
In `@src/jsc/bindings/bindings.cpp`:
- Around line 3944-3966: Update Bun__JSValue__isInstanceOfError to recognize
transparent proxies by inspecting their target directly, without invoking proxy
traps, and apply the existing ErrorInstanceType, JSDOMException, and
Error.prototype checks to that target. Preserve the current behavior for
non-proxy values and ensure proxy-wrapped errors return true.
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S3File.writer().end(error) ignored its argument. It flushed the buffered bytes, sent CompleteMultipartUpload (or the single PUT) and resolved, so a truncated object was stored under the key. end(error) with a value that is instanceof Error now fails the upload the same way a failed source stream of s3.write() does: queued parts are cancelled, AbortMultipartUpload is sent when S3 has returned an upload id, and nothing is sent when the upload has not started. The returned promise and a pending flush() reject with the caller's error. Both rejections are marked handled, because the caller already has the error. Any other argument stays a clean end. The check reads the stored prototype chain and looks through a Proxy, so a DOMException, an ES5-style error such as the AxiosError of axios 1.6, and a Proxy of an Error count too.
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I re-reviewed the latest push and found no new bugs; the changes since my earlier review address the points I raised there. Because this adds a user-facing semantic to writer().end(error) (the PR itself asks whether end(error) is the surface maintainers want) and routes through raw-pointer sink teardown, a maintainer look is still worthwhile.
What was reviewed:
end_with_error_from_jsin streams.rs:flush_promiseis taken beforefail_from_js_pumpandthisis not touched after it; both rejections now usereject_as_handled;errstays rooted by the call frame across the abort.JSSink__isErrorValuein generate-jssink.ts:getPrototypeDirectandProxyObject::target()run no script; a revoked Proxy was examined as a null-target candidate and ruled out (revoke nulls the handler, not the target).- Sink.rs routing is gated on
HAS_END_WITH_ERROR_FROM_JS, so FileSink/ArrayBufferSink/HTTP sinks keep the clean end for any argument; tests cover in-flight abort, non-started upload, Proxy/DOMException/ES5-style errors, the non-error options-object case, and the unhandled-rejection subprocess.
Extended reasoning...
Overview
The PR gives S3File.writer().end(error) real semantics: when the argument is instanceof Error (checked natively via a new JSSink__isErrorValue helper emitted from src/codegen/generate-jssink.ts), the generated JSSink::js_end in src/runtime/webcore/Sink.rs dispatches to a new JsSinkType::end_with_error_from_js hook. Only NetworkSink (src/runtime/webcore/streams.rs) opts in via HAS_END_WITH_ERROR_FROM_JS; it takes the pending flush promise, calls the existing fail_from_js_pump (which flags the upload done and sends AbortMultipartUpload once an upload id exists), then rejects both the taken flush promise and a fresh returned promise with the caller's error, marked handled. Docs, .d.ts JSDoc, and a new describe.concurrent block in test/js/bun/s3/s3.test.ts with a local mock S3 round out the change.
Compared to the version I reviewed earlier, the new commit moves the error predicate out of bindings.cpp into the JSSink generator, unwraps Proxy targets and matches on inherits<ErrorPrototype> (so cross-realm ES5-style errors and DOMException qualify), and rejects the flush promise as handled rather than as an unhandled rejection. Those were the three items I raised; each is addressed in the code, not just the description.
Security risks
None specific to this change. The new C++ helper reads only stored prototypes and Proxy targets, so no user traps or getters run from native code. A revoked Proxy was investigated as a possible null dereference in the while loop; in JSC, revoke replaces the handler with null and leaves m_target intact, so target() stays non-null and the walk continues to the underlying object. The error value is not stored on the sink, which avoids the Strong-cycle concern the PR calls out.
Level of scrutiny
Moderate-to-high. The Rust side operates on *mut NetworkSink across fail_from_js_pump, which can re-enter the sink through the S3 task's failure callback. I checked that after that call the function only touches the local flush_promise (already taken out of the sink) and the call-frame-rooted err, and that the ended early-exit defers to end_from_js so a second end(error) cannot double-abort. JSPromiseStrong::swap releases the Strong slot before rejecting, matching how on_writable settles the same promise. The remaining reason for a human look is design rather than correctness: the PR itself asks a maintainer whether end(error) is the intended surface versus a separate fail(), and it deliberately leaves close(error), FileSink.end(error), and the in-flight CreateMultipartUpload gap untouched.
Other factors
Test coverage is solid for a change of this size: an in-flight multipart abort with exact request ordering, the not-yet-started case across four error shapes (including Proxy and ES5-style), the non-error options-object case that node:stream/iter relies on, and a spawned-process check that unawaited end(error)/flush() do not exit the process. The tests use port: 0, using for the server, promise-based waits, and concurrent pipe draining. The bug hunt exited on a dry streak with two candidates ruled out, one of them a duplicate of the other.
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Actionable comments posted: 3
- 🪄 Fix CodeRabbit comments on this PR
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Inline comments:
In `@docs/runtime/s3.mdx`:
- Line 182: Update the S3 error-handling documentation and the corresponding
`packages/bun-types/s3.d.ts` declaration text to state that `end(error)` does
not create or replace an object: aborting the multipart upload removes only its
incomplete parts, while any existing object under the key remains unchanged.
In `@src/runtime/webcore/streams.rs`:
- Line 2615: Update MultiPartUpload::fail so in-flight UploadPart operations are
allowed to settle before final multipart cleanup, or trigger a follow-up
rollback_multi_part_request after the last active part completes. Ensure late
part responses still result in cleanup rather than only releasing local
resources, and update the test to release the held part after the initial abort
and assert the subsequent cleanup.
- Around line 2585-2623: Update the upload path around process_buffered and
end_with_error_from_js so a failed writer cannot leave an in-flight direct PUT
able to commit. Replace the SinglefileStarted direct-PUT flow with the
rollback-capable multipart upload protocol, ensuring failure invokes the
existing rollback behavior before completion can store the object; otherwise
revise the abort contract and related documentation to remove the no-object
guarantee.
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AbortMultipartUpload discards only the parts of the upload. An object that is already under the key stays as it is.
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A note on the merge order with #43140 (fixes #43133). The two diffs merge without a conflict, but they interact. #43140 adds an arm to
const writer = client.file("obj").writer();
writer.write("hello");
await writer.end(new Error("boom")); // rejects with "boom"
await writer.end(); // rejects with UnknownError: ReadableStream ended with an error
await writer.end(); // resolves 0The second call reports the generic error of
} else if let S3UploadResult::Failure(err) = &result {
if !sink.done {
sink.fail_unreported(err);
}
}With the guard, the second |
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We ran into this as users. A copy tool streams objects between two R2 buckets through The minimal repro below uses a local fake S3 and dummy credentials. On the released Bun 1.4.2 (darwin-arm64), it prints the same result in 3 of 3 runs: The expected sequence is repro (bun run repro.ts)// Bun S3 writer: end(error) after some data has been written.
// Expected: the multipart upload is aborted (DELETE ?uploadId=...), no object is created.
const MB = 1024 * 1024;
const requests: string[] = [];
const server = Bun.serve({
port: 0,
async fetch(req) {
const url = new URL(req.url);
const op = url.searchParams.has("uploads") ? "CreateMultipartUpload"
: url.searchParams.has("partNumber") ? "UploadPart"
: req.method === "DELETE" ? "AbortMultipartUpload"
: req.method === "POST" ? "CompleteMultipartUpload" : req.method + " (single-part)";
requests.push(op);
await req.arrayBuffer();
if (op === "CreateMultipartUpload") return new Response("<InitiateMultipartUploadResult><UploadId>u1</UploadId></InitiateMultipartUploadResult>");
if (op === "UploadPart" || op.startsWith("PUT")) return new Response(null, { headers: { etag: '"e"' } });
if (op === "AbortMultipartUpload") return new Response(null, { status: 204 });
return new Response("<CompleteMultipartUploadResult><ETag>\"done\"</ETag></CompleteMultipartUploadResult>");
},
});
const s3 = new Bun.S3Client({ accessKeyId: "test", secretAccessKey: "test", endpoint: `http://127.0.0.1:${server.port}`, bucket: "bucket", region: "auto" });
const writer = s3.file("object.bin").writer({ partSize: 5 * MB });
for (let i = 0; i < 7; i++) {
writer.write(new Uint8Array(MB));
await writer.flush();
}
const outcome = await Promise.race([
Promise.resolve(writer.end(new Error("the producer failed"))).then(() => "end(error) resolved", (e) => "end(error) rejected: " + e),
Bun.sleep(5000).then(() => "end(error) did not settle within 5 s"),
]);
await Bun.sleep(500);
console.log(Bun.version, "|", outcome, "|", requests.join(" -> "));
server.stop(true);
process.exit(0);For contrast, |
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Still reproduces on canary |
…riter-end-error-aborts
…ror) while CreateMultipartUpload is in flight clippy denies undocumented_unsafe_blocks, and the second unsafe block of NetworkSink::end_with_error_from_js had no SAFETY comment. The allow(clippy::not_unsafe_ptr_arg_deref) on it is removed: the lint only checks exported functions. Since the upload id that arrives after fail() is aborted, end(error) while CreateMultipartUpload is in flight also ends in AbortMultipartUpload. The docs say so, and a test holds the answer to CreateMultipartUpload to check it. The mock now holds a request until the test releases it.
…ipartUpload row expect(promise).rejects waits for the promise on the spot. On a build where end(error) still completes the upload, that promise needs the mock's handler to continue, and the test stalled until the server's idle timeout. Promise.allSettled gives a plain diff there.
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Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.
Beyond the inline test nits, I also checked two things in this push and ruled them out: a revoked Proxy passed to end() does not crash the target() unwrap loop in JSSink__isErrorValue (JSC's revoke() nulls the handler, not the target), and the docs' "abort is sent when the upload id arrives" claim matches start_multi_part_request_result in src/runtime/webcore/s3/multipart.rs, which sends the rollback when fail ran while CreateMultipartUpload was in flight. The sink is also not touched after fail_from_js_pump in end_with_error_from_js, so the synchronous failure callback releasing a writer holder is safe.
Extended reasoning...
The change adds an error-aware end path for the S3 NetworkSink across the Rust sink trait, the generated JSSink C++, types, docs, and tests; it touches no auth, injection or data-exposure surface. The three concerns from the prior review (flush promise rejected unhandled, Proxy/cross-realm errors missed, helper placed in bindings.cpp) are visibly addressed in the current code. Not approving because this introduces new user-facing API semantics via unsafe Rust and FFI that a maintainer should weigh, and the thread contains third-party comments whose content and resolution cannot be verified here.
- "sends nothing": the request list was read before a request of the writer could reach the mock. A probe request goes first now. - end(error) after end(): a new row pins that the pending promise of end() comes back and the object is stored. - A string and an object that only looks like an error complete the upload, like the options object. - The multipart rows wait for the request that ends the upload, so a build that completes it shows a diff and not a timeout.
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@code-yeongyu Thank you for the report and the repro. I ran your script as posted, 3 runs on each build.
So main still stores the truncated object, and this branch aborts the upload and rejects with your error. The branch now has main merged, and your report is in the description as the reason for the change. One detail for your tool: the argument must be |
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I reviewed the latest push and found no bugs; the two test gaps raised last time (the "sends nothing" probe and the end(error)-after-end() case) are now covered by a3090971. A human look is still worthwhile because this settles a user-facing API contract (end(error) aborting vs. a separate fail()), and the .d.ts change falls under a CODEOWNER.
What was reviewed:
JSSink__isErrorValue: a revoked Proxy does not null-deref — JSC'sProxyObject::revokeclears only the handler, sotarget()stays non-null; nested Proxies unwrap in the loop, and a Proxy mid-chain ends the walk via its null structure prototype.end_with_error_from_js:flush_promiseis taken beforefail_from_js_pump, so the synchronouswrapper_callbackfromMultiPartUpload::failfinds no promise to reject and only detaches the task;writer_holdersgoes 2 to 1, so the box outlives the call. Both rejections usereject_as_handled.- CreateMultipartUpload-in-flight abort relies on the existing
failedcheck instart_multi_part_request_resulton main, so the new test is valid against this base.
Extended reasoning...
The diff adds an end(error) path for NetworkSink only: a generated C++ instanceof Error-style check that runs no script, a new JsSinkType hook dispatched from JSSink::js_end, and a NetworkSink implementation that reuses fail_from_js_pump and rejects the returned and pending-flush promises as handled, plus docs, JSDoc, and a mock-server test block. It touches no auth, crypto, or input-parsing surface; the raw-pointer re-entrancy in the sink mirrors the existing fail_from_js_pump pattern and the reference counts on the writer box balance. Deferral rather than approval because packages/bun-types/s3.d.ts is CODEOWNER-owned and the PR explicitly asks a maintainer whether end(error) is the intended abort surface, which is a design call for a human.
The hook was a bool const plus a trait method with a default body. The default body was compiled once for each of the six sinks that never take an error end. An Option<fn> const, like START_TAG, has no body to instantiate, and a sink cannot set the flag and miss the method.
Problem
S3File.writer().end(error)ignoreserror. It sendsCompleteMultipartUpload(or the singlePUT) and resolves. A truncated object is stored: 5242980 of 20971520 bytes in the repro. A user hit this in production (Notes).JSSink::js_end(src/runtime/webcore/Sink.rs). It never readsframe.argument(0), althoughs3.d.tsdeclaresend(error?: Error).Fix
js_endsends an argument that isinstanceof Errorto the newJsSinkType::END_WITH_ERROR_FROM_JS. OnlyNetworkSinksets it. Other sinks and arguments keep the clean end.NetworkSinkcallsfail_from_js_pump, as a failed stream ofs3.write(key, stream)does (streams: deliver the source error to native sinks on controller.close(error) #40669). It cancels queued parts and sendsAbortMultipartUpload, also for an upload id that arrives later (s3: abort an upload that can never finish (collected writer, Create after fail, 204 abort) #41688).flush()reject with the caller's error and are marked handled: the caller already has it.test/js/bun/s3/s3.test.ts, blocks3 writer().end(error). 6 of 11 tests fail on main. Self-reviewed: 3 concerns raised, 3 addressed (Notes).Background
writer()returns aNetworkSink, aJSSinkwhoseendis the genericjs_end. It feeds aMultiPartUpload.partSize, it sendsCreateMultipartUpload, oneUploadPartper part, thenCompleteMultipartUpload, which makes the object visible.AbortMultipartUploaddiscards the parts.fail(reason)method: it does not fix code that already callsend(error).Downsides
Errortoend()and wants the partial object now gets a rejection.end()of an S3 writer makes 1 more C++ call, with no allocation. The release binary grows by 1280 bytes of text (size, linux-x64).writer.close(error)completes the upload. A part in flight atend(error)can stay stored (s3: send AbortMultipartUpload after the parts in flight have reported back #43154).Notes
Question for a maintainer: is
end(error)the surface you want for this? Afail(reason)method can be added later without a conflict. It does not help code that already callsend(error).Where this comes from:
S3File.writer()and callswriter.end(err)when the source fails. A truncated object stayed under the destination key. Their repro printsend(error) resolved | CreateMultipartUpload -> UploadPart -> UploadPart -> CompleteMultipartUploadon a release build of main 5c6d4e2. On this branch it printsend(error) rejected: Error: the producer failed | CreateMultipartUpload -> UploadPart -> AbortMultipartUpload. Both are 3 of 3 runs.isAnyError, whichnapi_is_errorandexpect().toThrow()also use. This one adds a separate check in the JSSink code.Callers in the wild:
writer.end(error instanceof Error ? error : new Error(String(error)))with the comment "end(error) aborts the multipart upload instead of committing a partial object".writer.end(error)does not abort, and deletes the key after it. That delete can remove an object that existed before.Left out on purpose:
writer.close(error)still takes the clean end.closeis not in the types.generate-jssink.tsemits it for every sink and passes no reason. To forward the argument changes every sink type.FileSink.end(error)still ignoreserror. A file has no commit step to skip. Its JSDoc is unchanged.Behavior details:
instanceof Errorcounts, becausepipeToofnode:stream/iterends its writer with an options object, and that must stay a clean end. A string and an object that only looks like an error also stay a clean end.JSSink__isErrorValue,generate-jssink.ts) reads the stored prototype chain and runs no script. It matches theError.prototypeof any realm by its class, and it looks at the target of a top-level Proxy. So aDOMException, an ES5-style error (theAxiosErrorof axios 1.6), and a Proxy of an Error count. A Proxy in the middle of a prototype chain ends the walk, because a chain can be cyclic through a Proxy.end(error)after an earlierend(), or after the upload failed, returns whatend()returns. The first outcome stands. A test pins the first case.end(error)whileCreateMultipartUploadis in flight rejects at once. The upload id that arrives later is aborted: s3: abort an upload that can never finish (collected writer, Create after fail, 204 abort) #41688 added that tostart_multi_part_request_result. A test holds the answer toCreateMultipartUploadto check it.end(error)rejects before the abort request reaches S3. A repro that stops its mock right after the rejection does not see theAbortMultipartUpload.Strongthat nothing clears would keep both alive.end_with_error_from_jstakes the pendingflush()promise and rejects it itself.Error, a string, aDOMException, or an ES5-style error stores nothing and rethrows. A source that completes stores the object.Measurements:
size build/release/bunon linux-x64 builds (bun run build:release) of the sources of main 5c6d4e2 and of this branch at 293e060. Text 107635854 to 107637134 (+1280 bytes). Data and bss do not change. The stripped file keeps its size (107808480 bytes). The release build of the branch also givesCreateMultipartUpload, AbortMultipartUploadand a rejection for the repro of the status comment.js_endof aNetworkSinkcallsJSSink__isErrorValuewith argument 0. Forend()with no argument that is oneisObjectcheck.END_WITH_ERROR_FROM_JSis an associated const and isNonefor the other six sinks, so they get no new code injs_end.mordantcheck flagged that body: it was compiled for the six sinks that never use it. The hook is oneOption<fn>const now, likeSTART_TAG. Both versions give the same release size.Merge order with #43140 (open): both PRs touch what the writer does after a failure. The comment of 2026-09-18 on this PR has the details. The PR that lands second takes the
!sink.doneguard and a test row forend(error)thenend().Tests:
endedguard and put a clean end in place offail_from_js_pump. The four "sends nothing" rows, the "after an earlier end()" row, and the two multipart rows fail, each with a diff.Promise.allSettledand notexpect().rejects. The matcher waits for the promise on the spot, and on a build that completes the upload the test then stalls until the idle timeout of the mock.test/js/bun/s3/,test/js/bun/util/filesink.test.ts,test/js/bun/util/arraybuffersink.test.ts,test/js/bun/stream/direct-readable-stream.test.tsx. The new block also passes underBUN_JSC_validateExceptionChecks=1, and 8 of 8 repeat runs pass.cargo clippy -p bun_runtime --no-depsandcargo check -p bun_runtime --all-targetspass. Thecargo clippycheck was red on the earlier head b333080: oneunsafeblock inend_with_error_from_jshad noSAFETY:comment.s3-list-objects.test.tshas 1 or 2 tests that time out when the whole file runs on the local ASAN debug build. Each of them passes alone, the file passes on the release build, and the diff does not touch that code.filesink.test.tsonce in 9 runs (FileSink > pipe, the file is intest/flaky-tests.txt), ands3-stream-error-gc.test.tsonce in 12 runs (two 5 s timeouts of tests that spawn a process and use no writer).Related open PRs: #43140 (an upload failure with no pending
flush()/end()promise is dropped, andend()resolves 0) and #43154 (MultiPartUpload::failsends the abort while parts are still in flight and does not abort again).[human-review] gate passed · iteration 0 · 6 files touched
fails on main (without fix)
passes on PR (with fix)
diff hotspot
gate history · 1 passed · 0 rejected · iteration 0
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